Generally, the MPP core is used on SMPS and DC converters, has a high permeability and shows a small frequency loss, so that the energy loss of the applied apparatus should be reduced, and that the bulk of the apparatus should also be reduced.
Generally, the MPP core is manufactured based on the process of FIG. 1, and this will be described below in detail. That is, in order to manufacture the MPP core, first an alloy consisting of Ni (nickel), Mo (molybdenum) and Fe (steel) is melted in a furnace such as an electric furnace or the like. Then, ingots of a certain size are formed.
The alloy for manufacturing the MPP core has the composition of 1.6-4.0 wt % of Mo, and 78-83 wt % of Ni, the balance being Fe. The melting of the alloy is carried out by heating it to a temperature of over 1500.degree. C. for 1 hour or more.
Then the ingots which have been formed in the above described manner are heated to a temperature of over 500.degree. C. to carry out hot rollings by 3 passes or more, thereby manufacturing strips having a width of about 60 inches. Then the strips are quenched by using a cooling medium such as water.
The quenching is carried out for facilitating the crushing which follows, and for forming a disordered state in the atomic arrangement within the material. Therefore, the quenching conditions are controlled in view of the above purposes. Then the strips which have undergone through the quenching are crushed to a certain particle size, and are made to pass through a mesh sieve, so that particles bigger than a certain size should be removed, thereby completing the manufacture of the MPP core forming powder.
The generally accepted average size of the MPP core forming powder is about 50 .mu.m, and in sorting the powder of this size, the scale of the sieve is selected to be 120 meshes, so that particles of over 120 meshes should be removed. Then mica is mixed to the sorted powder, and then, the mixture is heated under a reducing atmosphere containing hydrogen to a temperature of 1170.degree.-1400.degree. F. Then the mixture is maintained at the same temperature for hour or more, and then, the mixture is cooled down to 300.degree. C. within the furnace. Then the mixture is quenched down to the room temperature.
The above described annealing is for relieving the stress and strain remaining in the crushed powder, and therefore, the annealing conditions are controlled in this view.
The powder which is heat-treated in the above described manner is coated with a ceramic for insulating the particles, and then, the powder is formed into the desired shape.
Here, in order to reduce the frictions between the particles and particles and between the compacted body and the molding die, Zn-stearate is mixed by less than 1% prior to the molding.
Then the burrs which have been formed during the molding are removed, and then, the molded body is heated to a temperature of about 1170.degree. F. under a reducing gas atmosphere containing hydrogen. It is maintained for over 0.6 hours, and then, is cooled within the furnace, thereby completing the annealing. Then the magnetic properties are checked, and then, in order to protect the core properties from the humidity and the external atmosphere, a polyester is coated on the surface of it, thereby completing the manufacturing of the MPP core.
The above described annealing is for relieving the stress and strain remaining in the molded body, and therefore, the annealing conditions are controlled in this view.
The conventional process for manufacturing the MPP core as described above involves too much complicated process steps, with the result that the work efficiency is lowered, that the manufacturing cost is increased, and that the productivity is lowered.
The conventional process obtains the MPP core forming powder by crushing, and therefore, the particles have irregular polyhedral shapes. Consequently, the molding density is low, with the result that the permeability of the MPP core is lowered.
Further, in the case of the conventional process, the powder particles have sharp corners, and therefore, the ceramic coating becomes non-uniform. In other words, the insulating coating of the powder particles becomes non-uniform, and therefore, problems occur to the frequency characteristics of the MPP core.
Further, superior techniques for manufacturing products of a smaller bulk and a lighter weight are in demand, and therefore, studies have been briskly made to meet the demand.